This invention relates to an improved process and apparatus for the fractionation of a gaseous mixture by rectification wherein liquid in the sump of the rectifying column is heated and partially vaporized; and simultaneously liquid is withdrawn from the bottom zone of the sump liquid bath and recycled into the sump liquid bath at a point above the withdrawal point.
One process of this type has been described in German Pat. No. 2,238,866. The sump liquid is heated and partially vaporized by a condenser-evaporator disposed in the sump of the rectifying column, said condenser-evaporator being partially immersed in sump liquid during the operation of the column. On the condensation side of the evaporator-condenser, reflux liquid is produced for the rectification. The German patent particularly relates to the removal of sump liquid from the column in order, for example, to remove undesirable components from the sump liquid, and to the recycling of the resultant purified liquid to the pump. To avoid the use of recirculating pumps which otherwise not only result in expenditures in apparatus and energy, but also can prove hazardous, for example, in air fractionation for the recirculation of liquid oxygen, due to danger of explosion, the German patent discloses a system wherein the sump liquid is conveyed into a storage tank arranged above the sump liquid by means of the existing pressure difference between the liquid withdrawal point and the product discharge conduit. The stored liquid is eventually recycled into the column sump, utilizing the pressure gradient. More specifically, in a first switching phase, the storage tank is connected to the product conduit, and liquid is conveyed into the tank. In a second switching phase, the communication to the product conduit is disconnected, and the liquid is recycled from the storage tank into the column sump. Accordingly, with the use of the disclosed arrangement, liquid can be circulated without the utilization of a pump.
A disadvantage of this procedure is that the circulation does not take place continuously. Moreover, the storage tank and the required switching valves represent additional expenditure in apparatus and increase the space requirements for the plant.